Countless marine organisms were polluted with radioactive materials that were dispersed when the Fukushima Daiichi Nuclear Power Plant (FDNPP) was damaged in 2011 by the Great East Japan Earthquake. The aim of this study was to determine the degree to which marine herbivorous sea urchins, Mesocentrotus nudus, were contaminated with radiocesium because of the accident. We collected samples of sea urchins from four locations in Fukushima prefecture (at the coast and offshore from the Yotsukura and Ena stations) and investigated how the .sup.137 Cs activity concentrations changed. The biological half-life (T.sub.bio) of .sup.137 Cs in the individual sea urchins was between 121 and 157 days. The ecological half-life (T.sub.eco) of .sup.137 Cs was 181-423 days and was high in places close to the FDNPP. The T.sub.eco values in the sea urchins were longer than previously reported. The results infer that the food sources of the sea urchins around the Fukushima coast strongly influenced their uptake of .sup.137 Cs.